U.S. patent application number 09/826739 was filed with the patent office on 2002-10-10 for method and apparatus for the operation of a cell stack assembly during subfreezing temperatures.
Invention is credited to Ballinger, Emily A., Condit, David A., Yang, Deliang.
Application Number | 20020146608 09/826739 |
Document ID | / |
Family ID | 25247405 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020146608 |
Kind Code |
A1 |
Yang, Deliang ; et
al. |
October 10, 2002 |
Method and apparatus for the operation of a cell stack assembly
during subfreezing temperatures
Abstract
A coolant system is proposed for addressing temperature concerns
during start-up and shut-down of a cell stack assembly. The coolant
system comprises a coolant exhaust conduit in fluid communication
with a coolant exhaust manifold and a coolant pump, the coolant
exhaust conduit enabling transportation of exhausted coolant away
from a coolant exhaust manifold. A coolant return conduit is
provided to be in fluid communication with a coolant inlet manifold
and a coolant pump, the coolant return conduit enabling
transportation of the coolant to the coolant inlet manifold. The
coolant system further includes a bypass conduit in fluid
communication with the coolant exhaust conduit and the coolant
return conduit, while a bleed valve is in fluid communication with
the coolant exhaust conduit and a gaseous stream. Operation of the
bleed valve enables venting of the coolant from the coolant
channels, and through said bypass conduit.
Inventors: |
Yang, Deliang; (Vernon,
CT) ; Ballinger, Emily A.; (Vernon, CT) ;
Condit, David A.; (Avon, CT) |
Correspondence
Address: |
Melvin P. Williams
210 Main Street
Manchester
CT
06040
US
|
Family ID: |
25247405 |
Appl. No.: |
09/826739 |
Filed: |
April 5, 2001 |
Current U.S.
Class: |
429/429 |
Current CPC
Class: |
H01M 8/04022 20130101;
H01M 8/04253 20130101; H01M 8/04029 20130101; Y02E 60/50
20130101 |
Class at
Publication: |
429/26 |
International
Class: |
H01M 008/04 |
Claims
1. A coolant system for a cell stack assembly including a coolant
pump for circulating a coolant and coolant channels in fluid
communication with a coolant inlet manifold and a coolant exhaust
manifold, said coolant system comprising: a coolant exhaust conduit
in fluid communication with said coolant exhaust manifold and said
coolant pump, said coolant exhaust conduit enabling transportation
of exhausted coolant away from said coolant exhaust manifold; a
coolant return conduit in fluid communication with said coolant
inlet manifold and said coolant pump, said coolant return conduit
enabling transportation of said coolant to said coolant inlet
manifold; a bypass conduit in fluid communication with said coolant
exhaust conduit and said coolant return conduit; a bleed valve in
fluid communication with said coolant exhaust conduit and a gaseous
stream, wherein operation of said bleed valve enables venting of
said coolant from said coolant channels and through said bypass
conduit.
2. The coolant system for a cell stack assembly according to claim
1, wherein: said gaseous stream vents said coolant from said
coolant exhaust manifold, said coolant channels and said coolant
inlet manifold.
3. The coolant system for a cell stack assembly according to claim
1, further comprising: an inlet pressure control valve oriented
along said coolant return conduit and upstream of said bypass
conduit, wherein closing of said inlet pressure control valve
isolates said bypass conduit and said cell stack assembly from said
coolant circulated by said coolant pump.
4. The coolant system for a cell stack assembly according to claim
3, further comprising: a coolant exit valve oriented along said
coolant exhaust conduit between said bleed valve and said coolant
pump, wherein closing of said coolant exit valve directs said
gaseous stream through said coolant exhaust manifold, said coolant
channels and said coolant inlet manifold.
5. The coolant system for a cell stack assembly according to claim
3, further comprising: an accumulator oriented downstream from said
coolant pump for accumulating said coolant when said inlet pressure
control valve is closed.
6. The coolant system for a cell stack assembly according to claim
5, wherein: said accumulator is thermally insulated.
7. The coolant system for a cell stack assembly according to claim
6, wherein: said gaseous stream is an air stream; and said coolant
is one of a water solution and an antifreeze solution.
8. The coolant system for a cell stack assembly according to claim
7, wherein: said air stream is one of an ambient air stream and a
pressurized air stream.
9. A coolant system for a cell stack assembly including coolant
channels in fluid communication with a coolant inlet manifold and a
coolant exhaust manifold, said coolant system comprising: a coolant
pump for circulating a coolant stream, said coolant pump being in
fluid communication with said coolant inlet manifold and said
coolant exhaust manifold; a heater oriented downstream from said
coolant pump, between said coolant pump and said coolant inlet
manifold, for selectively heating said coolant stream; a bypass
conduit in fluid communication with said coolant inlet manifold and
said coolant exhaust manifold; and a bypass valve oriented along
said bypass conduit, wherein operation of said bypass valve permits
said coolant stream to pass from said coolant inlet manifold to
said coolant exhaust manifold, thereby avoiding substantial passage
through said coolant channels.
10. The coolant system for a cell stack assembly according to claim
9, wherein: said heater is selectively actuated to heat said
coolant stream when said bypass valve is opened.
11. The coolant system for a cell stack assembly according to claim
9, further comprising: an accumulator oriented downstream of said
coolant pump, between said coolant pump and said heater, for
accumulating excess quantities of said coolant stream.
12. The coolant system for a cell stack assembly according to claim
11, wherein: said accumulator is formed as an insulating component
for retaining a thermal energy of said coolant stream accumulated
therein.
13. A coolant system for a cell stack assembly, said coolant system
comprising: a coolant pump for circulating a coolant stream; a
heater for selectively heating said coolant stream externally of
said cell stack assembly; a burner for combusting residual fuel
exhausted from said cell stack assembly and outputting a heated
burner exhaust, said burner directing said heated burner exhaust to
be in thermal communication with said heater; and said coolant pump
directing said coolant stream through said heater, wherein said
coolant stream becomes warmed by said heated burner exhaust prior
to being introduced to said cell stack assembly.
14. A coolant system for a cell stack assembly according to claim
13, wherein: said heater includes an outer shell and an inner tube
portion; said burner directs said heated burner exhaust through
said inner tube portion; and said coolant pump directs said coolant
stream through said outer shell, thereby permitting thermal
communication between said inner tube portion and said outer
shell.
15. A method for protecting a cell stack assembly during periods of
subfreezing conditions, said method comprising the steps of:
opening a coolant bypass conduit, thereby enabling fluid
communication between an inlet side of a coolant inlet manifold and
an exhaust side of a coolant exhaust manifold; prohibiting a flow
of coolant into said coolant inlet manifold; and venting said
coolant flow from said coolant exhaust manifold, said cell stack
assembly and said coolant inlet manifold via said coolant bypass
conduit.
16. The method for protecting a cell stack assembly during periods
of subfreezing conditions according to claim 15, said method
further comprising the steps of: opening a bleed valve at said
exhaust side of said coolant exhaust manifold to enable said
venting; and maintaining operation of a coolant pump during a time
said bleed valve is opened.
17. The method for protecting a cell stack assembly during periods
of subfreezing conditions according to claim 15, said method
further comprising the steps of: prohibiting a flow of said coolant
from said coolant exhaust manifold to said coolant pump when said
coolant bypass conduit is opened.
18. A method for raising a temperature of a cell stack assembly
that has experienced subfreezing conditions, said method comprising
the steps of: enabling a coolant stream to pass from a coolant
inlet manifold to a coolant exhaust manifold without said coolant
stream passing through coolant channels in said cell stack
assembly; and heating said coolant stream prior to said coolant
stream being provided to said coolant inlet manifold.
19. The method for raising a temperature of a cell stack assembly
that has experienced inactivity in subfreezing conditions according
to claim 18, said method further comprising the steps of: storing
said coolant stream in an insulating accumulator upstream of said
coolant inlet manifold.
20. The method for raising a temperature of a cell stack assembly
that has experienced subfreezing conditions according to claim 18,
said heating step further comprising the steps of: directing a
heated burner exhaust through a first portion of a heat exchanger;
and directing said coolant stream through a second portion of said
heat exchanger.
21. The method for raising a temperature of a cell stack assembly
that has experienced subfreezing conditions according to claim 18,
said method further comprising the steps of: orienting a coolant
bypass conduit between said coolant inlet manifold and said coolant
exhaust manifold to enable said coolant stream to pass from said
coolant inlet manifold to said coolant exhaust manifold when said
coolant bypass conduit is opened.
22. The method for raising a temperature of a cell stack assembly
that has experienced subfreezing conditions according to claim 21,
said method further comprising the steps of: determining said
temperature of said cell stack assembly; and closing said coolant
bypass conduit when said temperature of said cell stack assembly is
determined to be above a predetermined temperature.
23. The method for raising a temperature of a cell stack assembly
that has experienced subfreezing conditions according to claim 20,
said heating step further comprising the steps of: directing said
heated burner exhaust through one of a fuel and an oxidant reactant
flow channels in said cell stack assembly after said heated burner
exhaust has been directed through said heat exchanger.
Description
FIELD OF THE INVENTION
[0001] This invention relates in general to a method and apparatus
for the operation of a cell stack assembly during subfreezing
temperatures, and deals more particularly with a method and
apparatus by which cell stack assemblies may avoid structural
damage to their constituent parts when experiencing harsh
environmental conditions, especially during times of operational
shut-down or start-up.
BACKGROUND OF THE INVENTION
[0002] Electrochemical fuel cell assemblies are known for their
ability to produce electricity and a subsequent reaction product
through the reaction of a fuel being provided to an anode and an
oxidant being provided to a cathode, thereby generating a potential
between these electrodes. Such fuel cell assemblies are very useful
and sought after due to their high efficiency, as compared to
internal combustion fuel systems and the like. Fuel cell assemblies
are additionally advantageous due to the environmentally friendly
chemical reaction by-products that are produced, such as water. In
order to control the temperature within the fuel cell assembly, a
coolant is provided to the fuel cell assembly. The coolant,
typically water, is circulated throughout the fuel cell assembly
via a configuration of coolant channels. The use of water within
fuel cell assemblies makes them particularly sensitive to freezing
temperatures.
[0003] Electrochemical fuel cell assemblies typically employ a
hydrogen-rich gas as the fuel and oxygen as an oxidant where, as
noted above, the reaction by-product is water. Such fuel cell
assemblies may employ a membrane consisting of a solid polymer
electrolyte, or ion exchange membrane, having a catalyst layer
formed thereon so as to promote the desired electrochemical
reaction. The catalyzed membrane is disposed between two electrode
substrates formed of porous, electrically conductive sheet
material--typically carbon fiber paper. The ion exchange membrane
is also known as a proton exchange membrane (hereinafter PEM), such
as sold by DuPont under the trade name NAFIONTM.
[0004] In operation, hydrogen fuel permeates the porous electrode
substrate material of the anode and reacts at the catalyst layer to
form hydrogen ions and electrons. The hydrogen ions migrate through
the membrane to the cathode and the electrons flow through an
external circuit to the cathode. At the cathode, the
oxygen-containing gas supply also permeates through the porous
electrode substrate material and reacts with the hydrogen ions, and
the electrons from the anode at the catalyst layer, to form the
by-product water. Not only does the ion exchange membrane
facilitate the migration of these hydrogen ions from the anode to
the cathode, but the ion exchange membrane also acts to isolate the
hydrogen fuel from the oxygen-containing gas oxidant. The reactions
taking place at the anode and cathode catalyst layers may be
represented by the equations:
Anode reaction: H.sub.2.fwdarw.2H.sup.++2e.sup.-
Cathode reaction: 1/2O.sub.2+2H.sup.++2e.sup.-.fwdarw.H.sub.2O
[0005] Conventional PEM fuels cells have the membrane electrode
assembly, comprised of the PEM and the electrode substrates,
positioned between two gas-impermeable, electrically conductive
plates, referred to as the anode and cathode plates. The plates are
typically formed from graphite, a graphite-polymer composite, or
the like. The plates act as a structural support for the two
porous, electrically conductive electrodes, as well as serving as
current collectors and providing the means for carrying the fuel
and oxidant to the anode and cathode, respectively. They are also
utilized for carrying away the reactant by-product water during
operation of the fuel cell.
[0006] When flow channels are formed within these plates for the
purposes of circulating either fuel or oxidant in the anode and
cathode plates, they are referred to as fluid flow field plates.
These plates may also function as water transfer plates in certain
fuel cell configurations and usually contain integral coolant
passages, thereby also serving as cooler plates in addition to
their well known water management functions. When the fluid flow
field plates simply overlay channels formed in the anode and
cathode porous material, they are referred to as separator plates.
Moreover, the fluid flow field plates may have formed therein
reactant feed manifolds, which are utilized for supplying fuel to
the anode flow channels or, alternatively, oxidant to the cathode
flow channels. They may also have corresponding exhaust manifolds
to direct unreacted components of the fuel and oxidant streams, and
any water generated as a by-product, from the fuel cell.
Alternatively, the manifolds may be external to the fuel cell
itself, as shown in commonly assigned U.S. Pat. No. 3,994,748
issued to Kunz et al. and incorporated herein by reference in its
entirety.
[0007] The catalyst layer in a fuel cell assembly is typically a
carbon supported platinum or platinum alloy, although other noble
metals or noble metal alloys may be utilized. Multiple electrically
connected fuel cell assemblies, consisting of two or more anode
plate/membrane/cathode plate combinations, may be referred to as a
cell stack assembly. A cell stack assembly is typically
electrically connected in series.
[0008] Recent efforts at producing the fuel for fuel cell
assemblies have focused on utilizing a hydrogen-rich gas stream
produced from the chemical conversion of hydrocarbon fuels, such as
methane, natural gas, gasoline or the like. This process produces a
hydrogen-rich gas stream efficiently as possible, thereby ensuring
that a minimal amount of carbon monoxide and other undesirable
chemical byproducts are produced. This conversion of hydrocarbons
is generally accomplished through the use of a steam reformer and
related fuel processing apparatus well known in the art.
[0009] As discussed previously, the anode and cathode plates may be
provided with coolant channels for the circulation of a water
coolant, as well as the wicking and carrying away of water produced
as a by-product of the fuel cell assembly operation. The water so
collected and circulated through a fuel cell assembly in the
coolant channels is susceptible to freezing below 32.degree. F.
(0.degree. C.) and may therefore damage and impair the operation of
the fuel cell assembly as the water expands when it freezes. It is
therefore necessary to provide a method and apparatus, which may
protect the fuel cell assembly during times of harsh environmental
conditions.
[0010] U.S. Pat. No. 5,798,186 issued to Fletcher et al. on Aug.
25, 1998 discloses various electrical heating configurations for
directly and indirectly thawing a fuel cell stack, which has
frozen. Additionally, mention is made as to having compliant or
compressible devices located within the stack manifold headers to
accommodate the expansion of freezing water within the fuel cell
stack. Such a system, localized only within the stack manifold
headers, will not fully protect the entirety of the fuel cell stack
or coolant channels from the effects of freezing and expanding
coolant.
[0011] In particular, there are those situations where the start-up
of the fuel cell assembly is desired after a time of inactivity in
subfreezing environmental conditions. In such cases it has been
discovered that attempting to circulate coolant through the coolant
channels, in order to alleviate the freezing conditions within the
fuel cell assembly, does not result in acceptable performance
characteristics. When, for example, water is utilized as the
coolant, the temperature of the fuel cell assembly typically causes
localized freezing at the input to the small-dimensioned coolant
channels, thereby partially blocking circulation therethrough and
unduly lengthening the time required for start-up. When non-porous
coolant channels or plates are utilized in conjunction with an
antifreeze solution coolant, similar problems exist due the high
viscosity of the antifreeze solution at low temperatures, again
lengthening the time required for start-up.
[0012] With the forgoing problems and concerns in mind, it is the
general object of the present invention to provide a fuel cell
assembly with a method and apparatus which overcomes the
above-described drawbacks even in times of subfreezing
temperatures.
SUMMARY OF THE INVENTION
[0013] It is an object of the present invention to provide a method
and apparatus for the operation of a cell stack assembly during
subfreezing temperatures.
[0014] It is another object of the present invention to provide a
shut-down procedure for a cell stack assembly which protects the
cell stack assembly even during times of subfreezing
temperatures.
[0015] It is another object of the present invention to provide a
start-up procedure for a cell stack assembly which protects the
cell stack assembly even during times of subfreezing
temperatures.
[0016] It is another object of the present invention to utilize a
thermally insulating coolant accumulator to assist in quickly
raising the temperature of the cell stack assembly.
[0017] According to one embodiment of the present invention a
coolant system is proposed for a cell stack assembly which includes
a coolant pump for circulating a coolant and coolant channels in
fluid communication with a coolant inlet manifold and a coolant
exhaust manifold. The coolant system comprises a coolant exhaust
conduit in fluid communication with the coolant exhaust manifold
and the coolant pump, the coolant exhaust conduit enabling
transportation of exhausted coolant away from the coolant exhaust
manifold. A coolant return conduit is provided to be in fluid
communication with the coolant inlet manifold and the coolant pump,
the coolant return conduit enabling transportation of the coolant
to the coolant inlet manifold. The coolant system further includes
a bypass conduit in fluid communication with the coolant exhaust
conduit and the coolant return conduit, while a bleed valve is in
fluid communication with the coolant exhaust conduit and a gaseous
stream. Operation of the bleed valve enables venting of the coolant
from the coolant channels, and through said bypass conduit.
[0018] These and other objectives of the present invention, and
their preferred embodiments, shall become clear by consideration of
the specification, claims and drawings taken as a whole.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic illustration of a coolant system,
according to one embodiment of the present invention.
[0020] FIG. 2 is a flow diagram illustrating the operation of the
coolant system in FIG. 1 during a shut-down procedure.
[0021] FIG. 3 is a flow diagram illustrating the operation of the
coolant system in FIG. 1 during a start-up procedure.
[0022] FIG. 4 is a schematic illustration of a coolant system,
according to another embodiment of the present invention
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0023] FIG. 1 illustrates a coolant system 100, according to one
embodiment of the present invention which may be operated to
protect a cell stack assembly 102 from the detrimental effects of
subfreezing temperatures during start-up and shut-down procedures.
As depicted in FIG. 1, the cell stack assembly (hereinafter `CSA`)
102 is comprised of a plurality of fuel cell assemblies 103 in
electrical communication with one another. The fuel cell assemblies
may each employ an ion exchange membrane consisting of a solid
polymer electrolyte disposed between an anode electrode substrate
and a cathode electrode substrate. An anode plate 107 and a cathode
plate 109 support reactant fuel channels 111 and reactant oxidant
channels 113, respectively. The ion exchange membrane may be a
proton exchange membrane (PEM) 105 comprising a polymer film
approximately 0.001 inch thick. The cathode and the anode electrode
substrates are typically formed of porous, electrically conductive
sheet material--typically carbon fiber paper having a Teflon.RTM.
coating. Coolant channels 104 are formed within typically porous
coolant plates, or the like, in each of these PEM fuel cell
assemblies 103, wherein water is typically utilized as the coolant
circulating through the coolant channels 104.
[0024] While PEM fuel cell assemblies have been described, the
present invention is not limited in this regard as other membranes
and electrode materials may be alternatively utilized, providing
they allow for the necessary flow of reactant and by-product
molecules, ions and electrons. In particular, fuel cell assemblies
utilizing an antifreeze solution circulating through coolant
channels in non-porous coolant plates may also be employed without
departing from the broader aspects of the present invention.
[0025] Still in reference to FIG. 1, a coolant inlet manifold 106
substantially evenly distributes a coolant to a plurality of
coolant channels 104, which are designed to uniformly circulate the
coolant about each of the fuel cell assemblies 103 comprising the
cell stack assembly 102. The coolant channels 104 are themselves
exhausted to a coolant exhaust manifold 108 after the coolant has
circulated through the cell stack assembly 102. Exhausted coolant
leaves the coolant manifold 108 via a coolant exhaust conduit 110
under the dynamic force of a coolant pump 112. The coolant is then
directed to an accumulator 114 prior to being shunted, with varying
ratios, to a heat exchanger 116 and an instantaneous heater 118, as
will be described in more detail later. A coolant return conduit
120 is subsequently provided to funnel the coolant once again to
the coolant inlet manifold 106.
[0026] When PEM fuel cell assemblies having porous coolant channels
or plates are utilized in the cell stack assembly 102, the coolant
circulating through the various components of FIG. 1 is maintained
at a subambient pressure by the coolant pump 112 and a coolant
inlet pressure control valve 122. By maintaining the coolant at
subambient pressures while adapting the reactants flows to be above
ambient pressures, the accumulation of liquid coolant in either the
fuel or the oxidant reactant streams is effectively avoided.
Moreover, the inclusion of the heat exchanger 116 provides a known
means to remove the heat absorbed by the circulating coolant prior
to the coolant being directed back to the cell stack assembly
102.
[0027] As described above, the coolant system 100 of FIG. 1 thereby
provides for the continuous supply and circulation of a coolant,
typically water, throughout the cell stack assembly 102 during
active operation thereof. While it should be readily apparent that
utilizing a water coolant within the cell stack assembly 102 is
beneficial for the purposes of water and thermal management,
problems arise when the cell stack assembly 102 experiences
temperatures at or below the freezing point of water; that is,
temperatures at or below 32.degree. F. (0.degree. C.). During times
when the cell stack assembly 102 experiences such temperatures, the
water contained within the cell stack assembly 102 begins to freeze
and expand, and may possibly cause damage to components of cell
stack assembly 102. It would therefore be very beneficial to equip
the cell stack assembly 102 with an apparatus which compensates for
the freezing of the water coolant and assuredly prevents
corresponding damage during times of shut-down and start-up.
[0028] It is therefore an important aspect of the present invention
to provide a method and apparatus for safely executing a shut down
procedure for the cell stack assembly 102 during times of
subfreezing temperatures. In known practices, when shut-down of the
cell stack assembly 102 is ordered, the water coolant is allowed to
drain from the cell stack assembly 102 under the force of gravity.
In effect, this means that the pressure differential between the
coolant supply and the reactant streams is no longer maintained by
the coolant pump 112 and the pressure control valve 122, hence, the
coolant will slump down into the reactant flow fields leaving a
portion of the cell stack assembly 102 immersed in a mixture of
water, fuel and oxidant. This condition may last indefinitely
during the shut-down period or, rather, may affect the cell stack
assembly 102 for a shorter time. In either case, damage may be
effected upon the cell stack assembly 102 during the time that the
water coolant is allowed to pool within the cell stack assembly 102
in an environment of subfreezing temperatures.
[0029] FIG. 2 illustrates a shut-down procedure 200 according to
one embodiment of the present invention which avoids the
above-described drawbacks and ensures that a shut-down operation of
the cell stack assembly 102 may be accomplished during subfreezing
temperatures without harm to the cell stack assembly 102. The
shut-down procedure 200 described herein is preferably begun after
the electrical load has been removed from the cell stack assembly
102, and after the reactant flows have been stopped and any
corrosion control steps have been completed.
[0030] The shut-down procedure 200 utilizes a shut-down bypass
conduit 124 and an influx of venting air, or the like. With
reference to FIGS. 1 and 2 in combination, the shut-down procedure
200 according to the present invention begins in step 202 by
initiating, either manually or automatically, a shut-down sequence.
As indicated in step 202, the coolant pump 112 continues to operate
after shut-down has been initiated in order to maintain the
subambient pressure within the coolant conduits. In this manner,
the present invention avoids the previously mentioned problem of
the coolant slumping in the reactant and coolant flow fields.
[0031] Returning to step 204 of the shut-down procedure 200 of FIG.
2, a shutdown valve 126 is opened in order to divert a substantial
portion of a coolant stream through the shut-down bypass conduit
124. A coolant exit valve 128, situated along the coolant exhaust
conduit 110, is then closed in subsequent step 206 in order to
prohibit the flow of coolant through the cell stack assembly 102.
In step 208 the cell stack assembly 102 is isolated from any
additional supply of coolant by closing the pressure control valve
122, while step 210 operates to open a bleed valve 130, thereby
placing the coolant system 100 in communication with an air supply.
In the preferred embodiment of the present invention, the bleed
valve 130 is in communication with an external ambient air supply
or atmosphere and serves to vent the coolant system by allowing
ambient air to be bled into the coolant conduits and flow fields.
As will be appreciated, the venting action is enabled by the
continued operation of the coolant pump 112 which maintains a
vacuum on the coolant conduits and flow fields.
[0032] While the present invention has been described as venting
the coolant conduits and flow fields with an ambient air supply,
alternative methods for evacuating the coolant from the coolant
conduits and flow fields may be employed without departing from the
broader aspects of the present invention. A pressurized source of
air may alternatively be placed in communication with the coolant
conduits and flow fields upon the opening of the bleed valve 130,
thus purging the coolant conduits and flow fields of any remaining
coolant.
[0033] As discussed above, by closing the various valves of the
coolant system 100 as described above, the air which is drawn
through the bleed valve 130 serves to vent the coolant exhaust
manifold 108, the coolant channels 104 and the coolant inlet
manifold 106 of any coolant remaining therein. The vented coolant
is directed through the shut-down bypass conduit 124 and eventually
deposited into the accumulator 114, leaving the reactant and
coolant channels in the cell stack assembly 102 free of
substantially all of the water coolant, although some water may
remain within the porous water transport plates.
[0034] During the venting process, it is determined in step 212
whether there still remains any coolant in the reactant and coolant
channels in the cell stack assembly 102. As long as coolant is
detected, the purging process continues as described above. When it
is determined that substantially no coolant remains in the cell
stack assembly 102, the shut-down bypass conduit 124 is closed and
the coolant pump 112 is disabled in step 214. The bleed valve 130
is subsequently closed in step 216 to end the purging process of
the shut-down procedure 200. As will be appreciated, various sensor
assemblies may be situated in the coolant exhaust manifold 108, the
coolant inlet manifold 106, or the coolant return conduit 120 to
determine if there remains any excess water coolant in the cell
stack assembly 102, in accordance with step 212.
[0035] The effect of the shut-down procedure 200 is to remove
substantially all of the coolant from the cell stack assembly 102,
thereby preventing the detrimental expansion of the coolant within
the cell stack assembly 102 during the period of time following
shut-down in subfreezing temperatures.
[0036] After shut-down, the cell stack assembly 102 faces the
related challenge of implementing a start-up command in subfreezing
temperatures. For practical concerns, including economics and
reliability, it is important that the cell stack assembly 102 begin
producing electricity as soon as possible after receiving a
start-up command. In addition, it is operationally critical that
the cell stack assembly 102 be capable of quickly circulating the
coolant and reactant flows immediately after start-up is initiated,
as damage to the cell stack assembly 102 may occur should a
significant time lag occur between these two events. It is
therefore an important aspect of the present invention to provide a
method and apparatus for a start-up procedure of the cell stack
assembly 102 during times of subfreezing temperatures.
[0037] FIG. 3 illustrates a start-up procedure 300 for ensuring
that a start-up operation of the cell stack assembly 102 may be
accomplished during subfreezing temperatures by utilizing a
start-up bypass conduit 132. It has been discovered that by
permitting a warmed coolant to flow through the inlet and exhaust
coolant manifolds, 106 and 108 respectively, it is possible to
quickly raise the temperature of the cell stack assembly 102 by
conduction, without the need for significant flow through the
coolant channels 104 themselves. As mentioned previously, by
substantially avoiding the coolant channels 104 during the initial
start-up of a subfreezing cell stack assembly, the formation of
frozen blockages in the coolant channels, and hence possible harm
to the cell stack assembly 102 as a whole, may be effectively
avoided.
[0038] With reference to FIGS. 1 and 3 in combination, the start-up
procedure 300 according to the present invention begins in step 302
by initiating, either manually or automatically, a start-up
sequence. The start-up sequence in step 302 includes activating the
coolant pump 112, as well as ensuring that the shutdown valve 126
is closed and the pressure control valve 122 is open. In step 304 a
bypass of the cell stack assembly 102 is accomplished by opening a
start-up valve 136 located along the start-up bypass conduit 132.
In this manner, coolant which is provided to the coolant inlet
manifold 106 is substantially entirely directed through the
start-up bypass conduit 132 and back into the coolant exhaust
manifold 108, thereby initially avoiding the coolant channels
104.
[0039] In step 306 the heat exchanger 116 is bypassed by opening
the heat bypass valve 138 situated along the heat bypass conduit
140. A thermostat-valve assembly 115 is utilized to ensure that no
coolant is permitted to flow through the heat exchanger 116 until
start-up of the cell stack assembly 102 has been accomplished
and/or the coolant temperature exceeds a predetermined
temperature.
[0040] Subsequent to opening the heat bypass valve 138 of FIG. 1,
the coolant circulated by the pump 112 will be directed through the
instantaneous heater 118, which is activated in step 308, for
quickly raising the temperature of the coolant provided to the
coolant manifolds, 106 and 108, respectively. As discussed above,
as the heated coolant is circulated through both the coolant inlet
manifold 106 and the coolant exhaust manifold 108 the cell stack
assembly 102 will quickly become heated due to heat conduction
stemming from the coolant manifolds, 106 and 108. A temperature
sensor 142 monitors the temperature of the cell stack assembly 102,
in step 310, to determine if the cell stack assembly 102 has risen
above a predetermined temperature T. Once the cell stack assembly
102 has risen above the predetermined temperature T, step 312 of
the start-up procedure 300 closes the start-up valve 136 and the
heat bypass valve 138, as well as shutting down the instantaneous
heater 118.
[0041] It will be readily appreciated that the predetermined
temperature T is preferably set as a temperature threshold which
would ensure that coolant provided to the coolant channels 104 will
not freeze and block the coolant channels 104. Most preferably, the
predetermined temperature T is set at approximately 32.degree. F.
or higher. Moreover, the temperature sensor 142 may be oriented at
various locations within the cell stack assembly 102, however,
orientation at a center-most location is preferable to ensure that
warming of the entirety of the cell stack assembly has been
substantially accomplished.
[0042] As described herein, the start-up procedure 300 is equally
applicable to PEM fuel cells which utilize a water coolant with
porous water transport plates, as well as for those fuel cells
which utilize an antifreeze coolant having nonporous water
transport plates.
[0043] Yet another important feature of the coolant system 100, as
depicted in FIG. 1, is the utilization of the accumulator 114 to
assist in start-up procedures. In accordance with the present
invention, the accumulator 114 is designed to be insulated so as to
keep the coolant deposited therein at elevated temperatures,
thereby assisting the start-up procedure 300 shown in FIG. 3. It
will be readily appreciated that the accumulator 114 may be a
thermos-type structure having thermally reflective components,
including multi-walled structures, or any alternative design
provided that the stored coolant retains significant thermal energy
for periods extending to several days or more.
[0044] FIG. 4 illustrates a coolant system 400 according to another
embodiment of the present invention. The coolant system 400 may be
utilized to quickly raise the temperature of a cell stack assembly
410 by heating the coolant stream provided to the cell stack
assembly 410. As depicted in FIG. 4, a burner 412 combusts a
residual fuel source, exhausted from unillustrated reactant fuel
flow fields of the cell stack assembly 410 via a fuel exhaust
conduit 411. This heated burner exhaust is subsequently exhausted
into a tube portion 414 of a shell and tube heat exchanger 420. In
conjunction with the heated burner exhaust being fed through the
tube portion 414, a shell 416 accepts a coolant stream therein so
as to promote a heat exchange between the burner exhaust and the
coolant stream. The newly heated coolant stream is subsequently
introduced into the cell stack assembly 410, resulting in an
increased rate of warming for the cell stack assembly 410.
[0045] To further increase the rate of heating the cell stack
assembly 410, the present invention further contemplates channeling
the heated burner exhaust, via heat conduit 418, to the anode
and/or cathode flow fields of the cell stack assembly 410. The
present invention also contemplates incorporating the coolant
system 400 of FIG. 4 into the coolant system 100 of FIG. 1, without
departing from the broader aspects of the present invention.
[0046] While the present invention describes combusting residual,
exhausted reactant fuel in the burner 412, the present invention is
not so limited as the burner 412 may be supplied with its own fuel
supply without departing from the broader aspects of the present
invention.
[0047] It is a major aspect of the present invention, therefore, to
provide a coolant system for a cell stack assembly which not only
provides protection against the destructive effects of subfreezing
temperatures during shut-down and cell stack inactivity, but also
operates to quickly raise the cell stack assembly above freezing
temperatures during a start-up procedure.
[0048] While the invention had been described with reference to the
preferred embodiments, it will be understood by those skilled in
the art that various obvious changes may be made, and equivalents
may be substituted for elements thereof, without departing from the
essential scope of the present invention. Therefore, it is intended
that the invention not be limited to the particular embodiments
disclosed, but that the invention includes all embodiments falling
within the scope of the appended claims.
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